The invention relates to a method for the optical recording of information in which a recording element which comprises a substrate and a recording layer provided thereon is exposed to laser light which is modulated in accordance with the information to be recorded, in which an optically readable change in structure occurs in the recording layer in the exposed places which is read by means of laser light via the substrate on the basis of differences in reflection with the surroundings.
Various systems for such optical recording are known. A system which is interesting for practical applications is the so-called ablative system. In this system a recording layer of, for example, Bi, a TeSe alloy or a dye is used in which holes or cavities are formed upon exposure to light. The holes or cavitites are read by means of weak laser light on the basis of differences in reflection between a hole or cavity and the surrounding thereof.
A practical disadvantage is that an air gap must be present above such an ablative recording layer. In practice, two ablative recording elements are interconnected, the recording layers facing each other, and spacers being used in order to provide an air(c.q. gas) gap between the recording layers. Hence the recording layer cannot be provided with a protective coating layer, for example, a coating lacquer layer.
A second system of optical recording is the phase change system. The recording layer used in this system is a layer of a semiconductor material, notably a TeSe alloy, to which various other elements, for example, As, Sb, S, may be added. Upon exposure to laser light, a change in structure occurs in the exposed places, amorphous information bits being formed in a crystalline layer, or conversely. The system is reversible so that, for example, by exposure to laser light, the amorphous information bits are converted again into crystalline material. The phase change system is known inter alia from U.S. Pat. No. 3,530,441. For practical application of this system, for example in the case of a Te-Se-Sb recording layer, the starting material is a crystalline layer in which amorphous bits are formed reversibly. The recording layer is provided on a supporting plate (substrate) by means of a sputtering process. The initially amorphous recording layer must first of all be converted into a crystalline layer by a heat treatment. This presents problems when a synthetic resin supporting plate (substrate) is used because the synthetic resins cannot withstand a heat treatment of, for example, 120.degree. C. for one hour. The recording layer must have a comparatively high crystallization temperature because otherwise the layer has insufficient stability and hence the stability of the recording element is restricted. The synthetic resin polymethyl methacrylate (PMMA) which, due to its low birefringence, is very suitable as such for use in a recording element is deformed and degraded by the temperature treatment. The synthetic resin polycarbonate acquires too large a birefringence as a result of the heat treatment. As a result of this the information recorded in the recording layer can no longer be read. Cross-linked synthetic resins, for example, the acrylate resins cross-linked by means of light, cannot withstand the above-mentioned temperature treatment either. The temperature treatment is an extremely critical process.
It is to be noted that reading takes place on the basis of difference in reflection of the reading laser light which is focused on the recording layer via the transparent substrate plate. The abovementioned disadvantage could possibly be avoided by not using the heat treatment and starting from an amorphous layer in which crystalline information bits are formed by exposure to radiation. However, this local crystallization is a slow process. In column 1, lines 60-65 of the abovementioned U.S. Pat. No. 3,530,441 a pulse duration of 1-100 milliseconds or more is mentioned to convert amorphous material locally into crystalline material.